Method and apparatus for substantially reducing electrical earth displacement current flow generated by wound components

ABSTRACT

An energy transfer element having an energy transfer element input winding and an energy transfer element output winding. In one aspect, the energy transfer element input winding is capacitively coupled to the energy transfer element output winding. The energy transfer element is capacitively coupled to electrical earth. One or more additional windings are introduced as part of the energy transfer element. The one or more additional windings substantially reduce capacitive displacement current between the energy transfer element input winding and energy transfer element output winding by balancing the relative electrostatic fields generated between these windings and/or between the energy transfer element and electrical earth by canceling the electrostatic fields generated by all windings within the energy transfer element relative to electrical earth through the selection of the physical position and number of turns in the additional windings.

RELATED APPLICATIONS

[0001] This application claims priority to U.S. provisional applicationserial No. 60/274,689, filed Mar. 8, 2001, entitled “Method andApparatus for Substantially Reducing Electrical Earth DisplacementCurrent Flow Generated by Wound Components.”

[0002] This application also claims priority to U.S. provisionalapplication serial No. 60/316,565, filed Aug. 31, 2001, entitled “Methodand Apparatus for Substantially Reducing Electrical Earth DisplacementCurrent Flow Generated by Wound Components.”

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates generally to energy transferelements and, more specifically, the present invention relates to energytransfer elements having multiple windings.

[0005] 2. Background Information

[0006]FIG. 1 shows an outline schematic diagram of a flyback powersupply 101. The basic operation of the flyback converter 101 is welldocumented and known to one skilled in the art. The primary switch 103is controlled through a feedback control signal 105, typically but notnecessarily from the secondary of the power supply as shown. The energytransfer element or transformer 107 windings have a dot polarity that isused to indicate the phase relationship of the winding voltages. Duringvoltage transitions across the windings, the dot end of the windings arein phase.

[0007]FIG. 2 is a schematic of a power supply 201, which expands on theoutline schematic of FIG. 1 by representing the parasitic capacitances209 that exist between the transformer core and electrical earth, theparasitic capacitances 211 that exist between the input and outputwindings and the transformer core and also the parasitic capacitances213 that exist between the input and output windings of the transformer.Usually the transformer core is the ferrite core used in the transformerconstruction to provide a low reluctance path for the magnetic fluxcoupling input and output windings of the transformer 207. As noted inFIG. 2, the parasitic capacitance 215 between the output of thetransformer and electrical earth in some cases maybe be short circuiteddepending on the application and or the way in which the electricalnoise measurements are made.

[0008] During the normal operation of the power supply 201, the voltagesacross both input and output windings of the transformer 207 transitionin accordance with the standard flyback power supply operation. Thesetransitions generate displacement currents in the electrical earththrough the various parasitic capacitances 209, 211, 213 and 215 shown.These displacement currents are detected as common mode noise (oremissions) and measured by a piece of test equipment called a Line InputStabilization Network (LISN). The configuration and connection of thisequipment is well documented and known to one skilled in the art.

[0009]FIG. 2 also highlights capacitor Cy 217 which is a Y-capacitor,that is commonly used in switching power supplies to reduce the commonmode emissions. This component, capacitor Cy 217, provides a lowimpedance path for displacement currents flowing between input andoutput windings of the transformer 207, to return to their sourcewithout flowing through electrical earth. The currents in capacitor Cy217 are not detected by the LISN and its use therefore acts to reducecommon mode emissions.

SUMMARY OF THE INVENTION

[0010] An energy transfer element having an energy transfer elementinput winding and an energy transfer element output winding isdisclosed. In one aspect, the energy transfer element input winding iscapacitively coupled to the energy transfer element output winding. Theenergy transfer element is capacitively coupled to electrical earth. Oneor more additional windings are introduced as part of the energytransfer element. The one or more additional windings substantiallyreduce capacitive displacement current between the energy transferelement input winding and energy transfer element output winding bybalancing the relative electrostatic fields generated between thesewindings and/or between the energy transfer element and electrical earthby canceling the electrostatic fields generated by all windings withinthe energy transfer element relative to electrical earth through theselection of the physical position and number of turns in the additionalwindings. Additional features and benefits of the present invention willbecome apparent from the detailed description and figures set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention detailed illustrated by way of example andnot limitation in the accompanying figures.

[0012]FIG. 1 is a schematic diagram of a flyback power supply.

[0013]FIG. 2 is a schematic diagram of a flyback power supply showingparasitic capacitances.

[0014]FIG. 3A is a schematic diagram of a transformer.

[0015]FIG. 3B is a cross section of a layer wound flyback transformer.

[0016]FIG. 4A is a schematic diagram of one embodiment of a transformerwound with a cancellation winding in accordance with the teachings ofthe present invention.

[0017]FIG. 4B is a cross section of one embodiment of a transformer witha cancellation winding in accordance with the teachings of the presentinvention.

[0018]FIG. 5A is a schematic diagram of one embodiment of a transformerwound with a balancing winding in accordance with the teachings of thepresent invention.

[0019]FIG. 5B is a cross section of one embodiment of a transformer witha balancing winding in accordance with the teachings of the presentinvention.

[0020]FIG. 6A is a schematic diagram of another embodiment of atransformer wound with a cancellation winding in accordance with theteachings of the present invention.

[0021]FIG. 6B is a cross section of another embodiment of a transformerwith a balancing winding in accordance with the teachings of the presentinvention.

[0022]FIG. 7A is a schematic diagram of yet another embodiment of atransformer in accordance with the teachings of the present invention.

[0023]FIG. 7B is a cross section of yet another embodiment of atransformer in accordance with the teachings of the present invention.

DETAILED DESCRIPTION

[0024] Embodiments of methods and apparatuses for reducing electricalearth displacement current flow generated by wound components aredisclosed. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one having ordinary skill inthe art that the specific detail need not be employed to practice thepresent invention. In other instances, well-known materials or methodshave not been described in detail in order to avoid obscuring thepresent invention.

[0025] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures or characteristics may be combined in any suitable manner inone or more embodiments.

[0026] Causes of electrical noise generated by switching power supplycircuits are well documented and known to those skilled in the art. Thisinvention specifically deals with the reduction in common mode noisegenerated by the energy transfer element, commonly referred to as thepower supply transformer, during the operation of a switching powersupply.

[0027] Since these techniques can be applied to flyback and forwardconverters, it is more accurate to refer to the transformer as theenergy transfer element. However in the specific embodiment discussedhere, a flyback circuit example is discussed and the energy transferelement is referred to as a transformer.

[0028] Various embodiments of the present invention described in hereinprovide techniques that are used in the construction of a transformer tosubstantially reduce the electrical earth currents generated by thepower supply allowing the system cost to be reduced either byeliminating the requirement to use a Y-capacitor or by reducing thevalue of Y capacitor necessary. Reducing the value of or eliminating theY capacitor also reduces leakage currents between the safety isolatedoutput and the AC input line. This is advantageous in applications wherethe output can come in contact with the user such as for example but notlimited to cellular phone applications or the like.

[0029] In particular, various embodiments of the techniques describedherein substantially reduce the capacitive displacement currents thatnormally flow in a switching power supply between the primary andsecondary windings, and the core of the transformer and electricalearth. In one embodiment, the reduction is achieved with the addition ofwindings in the transformer. In one embodiment, the number of turns ofthese additional windings are specifically selected based on the mainwinding turns to cancel and balance differential electrostatic fieldsgenerated by the main windings. These displacement currents normallyrequire that extra measures, such as for example in the form of externalcomponents, are taken in the design of the switching power supply toavoid these displacement currents interfering with other equipment.Various embodiments of the present invention therefore reduce systemcost by eliminating certain power supply components that would otherwisebe necessary to a designer not having the benefit of this disclosure.

[0030] As an overview, displacement currents generated by the operationof a switching power supply and flowing to electrical earth, aremeasured as electrical noise, also known as common mode emissions, thatcan cause electromagnetic interference (EMI) to other equipment. It istherefore necessary to maintain these currents below published limitsset up by regulatory bodies globally. Transformers in switching powersupplies generate displacement current flow to electrical earth in twoways.

[0031] One of the ways is the flow of displacement current between thecore of the transformer and electrical earth. This current is generatedby voltage transitions on the transformer windings coupling capacitivelyto the core of transformer. This current then flows capacitively throughfree space between the core of the transformer and electrical earth.

[0032] The other way is the flow of displacement current between theprimary and secondary windings of the transformer, which are set up bydifferential voltages between these windings. Differential voltagesbetween these windings generate current flow in the inter-windingcapacitance. This displacement current will return to its source throughparallel paths one of which is electrical earth.

[0033] Various embodiments of the present invention describe the use ofone or more additional windings within the transformer construction thatemploy the natural voltage fluctuations of the transformer windings tobalance and cancel the relative electrostatic fields between the inputand output windings that arise during the switching power supplyoperation. In one embodiment, the design of these additional windings isspecific to a particular transformer both in terms of the number ofturns used and their physical positioning. Through use of thesetechniques, the displacement current flow between the transformerwindings and transformer physical structure to electrical earth issubstantially reduced. This in turn eliminates or reduces the cost ofexternal components such as Y capacitors that are used to reduce commonmode emissions.

[0034] To illustrate, FIG. 3A shows a simple outline schematic of atransformer 301. The two ends of the input winding 303 are labeled nodesA and B. The two ends of the output winding 305 are labeled nodes C andD. For the purposes of this description, the physical core 307 of thetransformer is labeled as a further node E. The dot polarity of thewindings 303 and 305 is such that when there is a voltage transition onthe input winding 303 such that node B is becoming more positiverelative to node A, the voltage of node D will increase relative to nodeC.

[0035] As described above, these voltage transitions generatedisplacement currents in the parasitic capacitances resulting in currentflowing to electrical earth. As will be discussed, additional windingsare provided in one embodiment of the present invention to substantiallyreduce these electrical earth currents.

[0036] In particular, in one embodiment, a winding technique is used toreduce displacements currents between the transformer windings 303 and305 and transformer core 307. In the construction of the transformer301, one of the windings 303 or 305 is normally in closer proximity tothe transformer core 307 than the other. Furthermore one of the windings303 or 305 typically has higher voltage transitions across it.

[0037] For instance, FIG. 3B shows the typical cross section of a layerwound flyback transformer 301 where node B of input winding 303 is woundclosest to the transformer core 307. The output winding 305 is woundoutside the input winding 303 and therefore has less influence on thegeneration of displacement currents between windings 303 and 305 andtransformer core 307 since it is physically further from the transformercore 307.

[0038]FIG. 4A shows the schematic of one embodiment of a transformer 401wound with a cancellation winding 409 coupled to the primary inputwinding 403 since the placement of the primary input winding 403 in thiscase has most influence on the winding to transformer core 407displacement current.

[0039]FIG. 4B shows the cross section of one embodiment of thetransformer 401 with cancellation winding 407 between Nodes F and G. Asshown in the depicted embodiment, Node F is connected to node A and NodeG is left uncoupled electrically. In one embodiment, the dot polarity ofthe cancellation winding 409 is such that its electrostatic fieldopposes that created by the input winding 403.

[0040] In one embodiment, the exact choice of the number of turns usedin this cancellation winding 409 is determined based on empiricaloptimization techniques. In one embodiment, the variables consideredinclude the percentage of the overall primary winding voltage seen bythe first layer of the primary input winding 403, which is 33% in theembodiment illustrated in FIG. 4b since there are 3 layers. Howeversince the first layer of the input winding 403 is now further from thetransformer core 407 due to the presence of the cancellation winding409, its electrostatic field is weaker at the transformer core 407. Assuch, the cancellation winding 407 typically requires fewer turns thanthe first layer of the input winding 403 to provide cancellation.

[0041] In one embodiment, the exact cancellation is more complex sincethere are lesser influences from all windings 403, 405 and 409, hencethe reason that empirical techniques provide an effective optimization.In one embodiment, the net effect is that the influence of theelectrostatic field produced by the other windings in the transformerconstruction relative to the transformer core 407 are cancelled by theelectrostatic field created by the cancellation winding 409.Consequently the displacement current between transformer windings andtransformer core is theoretically zero if the electrostatic fieldsperfectly cancel. In practice, the effect is to substantially reduce thenet displacement current.

[0042] In another embodiment, a second winding technique is used toreduce the displacement current flowing between input winding and outputwinding. To illustrate, FIG. 5A shows the schematic of a transformer 501having this additional winding which is referred to as a balancingwinding 511 since its net effect is to balance the electrostatic fieldsgenerated between input and output windings 503 and 505 of thetransformer 501. The transformer 501 of FIG. 5A and the cross section oftransformer 501 illustrated in FIG. 5B show the balance winding 511 inconjunction with the cancellation winding 509 described above.

[0043] In the embodiment depicted in FIG. 5A, the balancing winding 511between nodes H and I is shown coupled to the input winding 503 with thedot polarity such that the node H is connected to node A. In theembodiment depicted in FIG. 5B, a cross section of transformer 501 isshown with the inclusion of the balancing winding 511. In otherembodiments balancing winding 511 is connected to the output winding505, as is the case with an embodiment described below, depending onwhich of the windings is the prime generator of the displacementcurrent. In a circuit where there are more output turns than inputturns, the output winding 505 could be seen as the prime generator inwhich case, the optimum connection of the balancing winding 511 could beby coupling to the output winding 505.

[0044] In the embodiment depicted in FIG. 5A, the primary input winding503 is assumed to be the prime generator and as such, the additionalbalance winding 511 is designed to oppose the electrostatic fieldgenerated by the input winding 503. In one embodiment, the number ofturns are selected such that the net electrostatic field from thecombination of the balancing and input windings 511 and 503, exactlymatches that generated by the output winding 505. When this is achieved,the differential field between primary and secondary circuits is zeroand the displacement current is also zero. In practice, the effect is tosubstantially reduce the net displacement current.

[0045] The embodiment illustrated in FIGS. 6A and 6B shows anothertechnique to substantially reduce the net displacement current betweenthe input and output windings 603 and 605 of the transformer 601 anddisplacement current that flows capacitively through free space betweenthe core of the transformer and electrical earth. In particular, FIG. 6Ais a schematic diagram of transformer 601 wound with a balancing winding611 and FIG. 6B shows a cross section of transformer 601 with balancingwinding 611 in accordance with the teachings of the present invention.With the technique as illustrated in FIGS. 6A and 6B, the balancingwinding 611 is positioned outside the output winding 605 in theconstruction of the transformer 601. The dot polarity of balancingwinding 611 is such that it opposes the electrostatic field generated bythe input winding 603 relative to the output winding 605.

[0046] Referring to the node designation in FIGS. 6A and 6B, Node D ofthe output winding 605 is positioned to reduce the relativeelectrostatic field between the main input winding 603 and the outputwinding 605. However, in the embodiment shown, the input winding 603 has3 layers and therefore 33% of the voltage across the primary inputwinding 603 is seen in the outer most layer next to the output winding605. Since, in a typical design, the output winding 605 has fewer turnsthan the outer layer of the primary input winding 603, the input winding603 is the primary generator of displacement current. By electricallycoupling Nodes A and F, Node G opposes the electrostatic field generatedby the input winding 603. The correct choice of number of turnstherefore substantially reduces the net displacement current betweeninput and output windings 603 and 605.

[0047] The position of the balancing winding 611 on the outside of theother windings in the transformer construction also means that itselectrostatic field opposes that of the input winding 603 relative tothe physical core 607 of the transformer 601. In this way the balancingwinding 611 in this embodiment also provides a degree of cancellation ofcapacitively coupled displacement current through free space between thecore of the transformer and electrical earth, previously provided by theseparate cancellation winding 409 of FIG. 4.

[0048] This single winding technique also provides the advantage ofreducing the leakage inductance between the input and output windings603 and 605 since these two windings are physically closer in thetransformer construction.

[0049] The overall effectiveness of this single winding technique toprovide the cancellation functions described above is dependent on thephysical nature of the particular transformer and the ability toposition the winding optimally with respect to both the output winding605 and the physical core 607 of the transformer 601. As such, theembodiment described below employs the two winding technique describedearlier.

[0050]FIG. 7A shows specific details of a schematic and cross section ofone embodiment of a transformer 701 using the techniques described abovein accordance with the teachings of the present invention. FIG. 7B showsa cross-section of one embodiment of transformer 701 in accordance withthe teachings of the present invention. This design of transformer 701has a cancellation winding 709 between the main input winding 703(primary) and the transformer core 707. In addition, this design uses abias winding 713 as the low voltage supply for the power supplyswitching regulator circuitry coupled to the input winding 703. Thebalancing winding 711 in this case is coupled to the output winding 705since, in this particular design, the output winding 705 is the primarygenerator of common mode displacement currents flowing between theprimary and secondary input and output windings 703 and 705. Accordingto the description above therefore, this coupling of the balancingwinding 711 provides the optimum performance.

[0051] Tables I, II, III and IV below summarize the electricalspecifications, materials, winding instructions and winding circuitconnections associated with one embodiment of transformer 701 inaccordance with the teachings of the present invention. TABLE IElectrical Specifications. Electrical 60 Hz 1 minute, from Pins Strength1-5 to Pins 6-10 3000 Vac Creepage Between Pins 1-5 and 6.2 mm (Min.)Pins 6-10 Primary Inductance All windings open 980 uH +/− 10% ResonantFrequency All windings open 500 KHz (Min.) Primary Leakage Pins 6-10shorted 35 uH Inductance

[0052] TABLE II Materials. Item Description [1] Core: EE19 Gapped for ALof 170 nH/T² [2] Bobbin: YC-190Z EE-19 [3] Magnet Wire: #33 AWG HeavyNyleze [4] Triple insulated wire: #26 AWG Heavy Nyleze [5] Magnet Wire:#28 AWG Heavy Nyleze [6] Tape: 3M 1298 Polyester Film (white) 9.0 mmwide by 2.2 mils thick [7] Varnish

[0053] TABLE III Winding Instructions. Cancellation Start at Pin 2. Windone layer from left to right. Cover Winding with one layer of Mylartape. Lay end of wire over tape and cover with another layer of tape.Input Start at Pin 5. Wind 95 turns of item [3] from left to right.Winding Wind in 3 layers. Apply 1 layer of tape, item [6], for basicinsulation between each layer. Finish on Pin 2. Basic 1 layer of tape[6] for basic insulation. Insulation Bias Start at Pin 3. Wind 15bifilar turns item [5] from left to Winding right. Wind uniformly, in asingle layer, across entire width of bobbin. Finish on Pin 4. Coverwinding with one layer of tape and layover end of wire. Basic 1 layer oftape [6] for basic insulation. Insulation Balancing Start with item #4taped and unterminated and wind five Winding turns evenly left to right.Finish on pin 10. Output Start at Pin 10. Wind 6 turns of item [4]bifilar from left to Winding right. Wind uniformly, in a single layer,across entire width of bobbin. Finish on Pin 6. Final Assemble andsecure core halves. Impregnate uniformly Assembly [7].

[0054] TABLE IV Winding Circuit Connections. Pin 1 No Connection Pin 2Primary DC Input Pin 5 Drain (or Switching Element) Connection Pin 3Bias Output Pin 4 Bias Return Pin 6 Output Return Pin 10 Output

[0055] Note that in one embodiment, the balancing winding dot phase isunterminated.

[0056] In the foregoing detailed description, the method and apparatusof the present invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. An energy transfer element, comprising: an energytransfer element core; a first winding wound around the energy transferelement core; a second winding wound around the energy transfer elementcore, the first winding capacitively coupled to the second winding; anda third winding wound around the energy transfer element core, the thirdwinding to generate a third winding electrostatic field to substantiallycancel relative electrostatic fields generated by the first and secondwindings relative to the energy transfer element core to substantiallyreduce a capacitive displacement current between the first and secondwindings.
 2. The energy transfer element of 1 wherein the third windingis electrically coupled to the first winding.
 3. The energy transferelement of 1 wherein the third winding is electrically coupled to thesecond winding.
 4. The energy transfer element of claim 1 wherein thefirst and second windings are capacitively coupled to electrical earth,wherein the third winding electrostatic field substantially cancelsrelative electrostatic fields generated by the first and second windingsrelative to the energy transfer element core to substantially reduce acapacitive displacement current between the energy transfer element coreand electrical earth.
 5. The energy transfer element of claim 1 whereinthe third winding is wound around the energy transfer element core witha number of turns based at least in part on a function of a percentageportion of the first winding included in a first layer of the firstwinding.
 6. The energy transfer element of claim 1 wherein the firstwinding is physically wound closer to the energy transfer element corethan the second winding.
 7. The energy transfer element of claim 6wherein the third winding is physically wound closer to the energytransfer element core than the first winding.
 8. The energy transferelement of claim 6 wherein the first and second windings are physicallywound closer to the energy transfer element core than the third winding.9. The energy transfer element of claim 1 wherein the energy transferelement is included in a flyback transformer.
 10. The energy transferelement of claim 1 wherein the energy transfer element is included in aforward converter transformer.
 11. The energy transfer element of claim1 wherein the first winding is an input winding of the energy transferelement and the second winding is an output winding of the energytransfer element.
 12. The energy transfer element of claim 1 furthercomprising a fourth winding wound around the energy transfer elementcore between the first and second windings to substantially reduce thecapacitive displacement current between the first and second windings,the fourth winding coupled to one of the first and second windings. 13.The energy transfer element of claim 12 wherein the first winding is aninput winding of the energy transfer element and the second winding isan output winding of the energy transfer element.
 14. The energytransfer element of claim 12 wherein the first winding is an outputwinding of the energy transfer element and the second winding is aninput winding of the energy transfer element.
 15. The energy transferelement of claim 1 wherein the energy transfer element is included in apower supply.
 16. An energy transfer element, comprising: an energytransfer element core; a first winding wound around the energy transferelement core; a second winding wound around the energy transfer elementcore, the first and second windings capacitively coupled to electricalearth; and a third winding wound around the energy transfer elementcore, the third winding to generate a third winding electrostatic fieldto substantially cancel relative electrostatic fields generated by thefirst and second windings relative to the energy transfer element coreto substantially reduce a capacitive displacement current between theenergy transfer element core and electrical earth.
 17. The energytransfer element of 16 wherein the third winding is electrically coupledto the first winding
 18. The energy transfer element of 16 wherein thethird winding is electrically coupled to the second winding
 19. Theenergy transfer element of claim 16 wherein the first windingcapacitively is coupled to the second winding, wherein the third windingelectrostatic field substantially cancels relative electrostatic fieldsgenerated by the first and second windings relative to the energytransfer element core to substantially reduce a capacitive displacementcurrent between the first and second windings and the energy transferelement core.
 20. The energy transfer element of claim 16 wherein thethird winding is wound around the energy transfer element core with anumber of turns based at least in part on a function of a percentageportion of the first winding included in a first layer of the firstwinding.
 21. The energy transfer element of claim 16 wherein the firstwinding is physically wound closer to the energy transfer element corethan the second winding.
 22. The energy transfer element of claim 21wherein the third winding is physically wound closer to the energytransfer element core than the first winding.
 23. The energy transferelement of claim 21 wherein the first and second windings are physicallywound closer to the energy transfer element core than the third winding.24. The energy transfer element of claim 16 wherein the energy transferelement is included in a flyback transformer.
 25. The energy transferelement of claim 16 wherein the energy transfer element is included in aforward converter transformer.
 26. The energy transfer element of claim16 wherein the first winding is an input winding of the energy transferelement and the second winding is an output winding of the energytransfer element.
 27. The energy transfer element of claim 16 whereinthe first winding is an output winding of the energy transfer elementand the second winding is an input winding of the energy transferelement.
 28. The energy transfer element of claim 16 wherein the energytransfer element is included in a power supply.
 29. A flyback powersupply, comprising: first and second input voltage terminals; and anenergy transfer element having a first winding and a second windingwound around an energy transfer element core, a first end of the firstwinding coupled to the first input voltage terminal, the first windingcapacitively coupled to the second winding, the energy transfer elementcore capacitively coupled to electrical earth, the energy transferelement further including a third winding wound around the energytransfer element core, the third winding to generate a third windingelectrostatic field to substantially cancel electrostatic fieldsgenerated by the first and second windings relative to each other andelectrostatic fields generated by the first and second windings relativeto the energy transfer element core to substantially reduce a capacitivedisplacement current between the first and second windings and tosubstantially reduce a capacitive displacement current between theenergy transfer element core and electrical earth.
 30. The power supplyof claim 29 further comprising a switch having a first terminal coupledto a second end of the first winding, the switch having a secondterminal coupled to the second input voltage terminal, the switch havinga third terminal coupled to receive a feedback control signal, theswitch controlled through the feedback control signal.
 31. The flybackpower supply of claim 29 wherein the third winding is wound around theenergy transfer element core with a number of turns based at least inpart on a function of a percentage portion of the first winding includedin a first layer of the first winding.
 32. The flyback power supply ofclaim 29 wherein the first winding is physically wound closer to theenergy transfer element core than the second winding.
 33. The flybackpower supply of claim 32 wherein the third winding is physically woundcloser to the energy transfer element core than the first winding. 34.The flyback power supply of claim 32 wherein the first and secondwindings are physically wound closer to the energy transfer element corethan the third winding.
 35. The flyback power supply of claim 29 whereinthe energy transfer element further includes a fourth winding woundaround the energy transfer element core between the first and secondwindings to substantially reduce the capacitive displacement currentbetween the first and second windings, the fourth winding coupled to oneof the first and second windings having a greater number of turns.